This invention is in the field of heat transfer; and more particularly, the invention relates to internal cooling of heat exchanger tubes.
The present invention has particular aplication in batch coil annealing furnaces. The background of this invention will be more clearly understood by having reference to FIG. 1 which is a sectional view of a batch coil annealing furnace 10 and FIG. 2 which is a sectional view of a base 15 of the batch coil annealing furnace 10. Illustrated in FIG. 1 are metal coils 11 stacked on bases 15 within atmosphere protecting inner covers 12 contained within outer cover 9 of furnace 10.
In the batch coil annealing process for heating a charge of steel coils, the charge is heated according to a predetermined or controlled temperature time relation and then cooled. After the heating, the outer cover 9 is removed from the furnace 10 and the charge is then cooled. In the usual non-accelerated method of cooling, heat from the charge is transferred by convection and radiation to the surrounding inner cover 12. The inner cover in turn loses heat by convection and radiation to its surroundings. Forced convection of the atmosphere within the inner cover is provided by the base fan 19. Enhanced convective heat transfer on the outside of the inner cover can also be provided by fans or fan-type cooling hoods over the inner covers. This cooling method is slow with heat removal from the inner cover being the barrier to heat removal.
The cooling can be accelerated by using a system of heat exchangers located in the base 15 of each stand. FIG. 2 is a sectional view of the base along section 2--2 of FIG. 1. A fan is centrally mounted within the base. Concentrically mounted about the fan are two banks of heat exchanger tubes, such as Intra-Kool.RTM. (trademark registered to Midland-Ross Corporation) heat exchanger tubes, an inner tube bank 20 and an outer tube bank 21. Between the base fan 19 and the inner tube bank 20 are diffuser shape supports 24 and between the two banks of cooling tubes are additional supports, such as the circular supports 25. A deflector ring 22 can be concentrically mounted about the outer tube bank 21.
Heat exchanger tubes, such as Intra-Kool.RTM. tubes for batch coil annealing furnace bases come in standard lengths for the inner tube banks 20 and standard lengths for the outer tube banks 21. All Intra-Kool.RTM. tubes have a 3/4 of an inch ID. The two of the most widely used tube materials are 316 L stainless steel and nickel based alloy 600 (75% nickel, 15% chromium).
During the heating step, the heat exchanger tubes are dry with outside surfaces exposed to hot circulating gas within the inner covers 12. The inside surfaces are exposed to non-circulating air. During the cooling step, air or water is circulated within the heat exchanger tubes. The tubes are not costly to install and provide very rapid cooling, particularly when cooling water circulated, but the tubes distort badly and crack after several months of operation. If the tubes crack, water from within the tube can damage the charge. Total tube replacement is usually necessary in a short time which can be as short as a hundred heats in extreme cases.
The primary reason for failure of these tubes is thermal shock. Thermal shock as defined means that the temperature gradient across the thickness of the tube wall, around the circumference, or along the length, would produce a thermal stress in excess of the short time yield strength of the tube material.
Rapid cooling cycles used in the past have sent quenching water through the heat exchanger tubing when the tubes were at temperatures above 1300.degree. F. Quench water added at this temperature results in a severe thermal shock and steam formation. This imparts sufficient forces through the tube to cause it to move violently.
In addition to the use of water or air as a coolant in the heat exchanger tube banks, U.S. Pat. No. 3,140,743 by C. Cone discloses a cooling method for use in the heat exchanger tubes in the base of batch coil annealing furnaces. Initially, air is supplied through the tubes until a temperature of about 800.degree. F. At this time the air line is turned off and a water supply is turned on. Cone goes on to suggest that a mist of atomized water can be provided between the air and the water circulation steps of the cooling cycle. Although Cone suggests the use of a mist of atomized water, he does not disclose or suggest how to accomplish the injection of the atomized water. Nor does Cone suggest operating parameters for changes in the different steps going from air to atomized water to water.
One method of introducing water mist into the heat exchanger tubes used in batch coil annealing furnaces has been to first allow the system to cool down to approximately 1000.degree. F. naturally and then add a water mist for a period of about 5 minutes. This is followed by water alone. The method, to date, however, uses one water mist nozzle to supply water mist to a common manifold. The water mist then goes to all of the heat exchanger tubes. This method results in poor mist distribution to individual tubes and within individual tubes.
It is desireable to have an apparatus and a method of operation of cooling heat exchanger tubes, such as Intra-Kool.RTM. tubes, in batch coil annealing furnaces wherein control can be had over the air, mist and water entering each individual tube. Such an apparatus and method is important when considering: thermal stresses along the length, across the walls and circumferentially around the heat exchanger tubes; as well as the more rapid rate of cooling which air, water mist and water will result in when used in heat exchanger tubes.